1. Field of the Invention
This invention pertains to the general field of sublimation or heat transfer printing, whereby an image is transferred to a receptor surface from a sublimable transfer print. In particular, the invention concerns a new polyurethane composition and method for coating ceramics and other surfaces to improve the quality and durability of the transferred image.
2. Background of the Invention
Sublimation or thermal transfer printing is a relatively new technology that has found its way into consumer product application during the past decade. In particular, the market for personalized decoration of clothing such as T-shirts and coffee mugs has grown explosively during the past five years.
Basically, the process of imprinting an article via this process consists of converting a picture into a digital image and transferring it onto a diffusable or sublimable transfer print. The dyes used for the transfer print are capable of diffusing or sublimating from the transfer print into another receptive surface when the two surfaces are brought into intimate contact, normally under pressure, and heated at specific operating conditions. Typically, heating is carried out at 375 to 500 degrees Fahrenheit (190.degree.-260.degree. C.) for 60 seconds to 3 minutes, the specific length of time depending on factors such as how fast the heater is able to bring the receiving surface to the temperature at which the dyes diffuse or sublimate and the temperature at which the transfer print is able to release the dyes. With currently used dyes, this temperature is about 375.degree. F. (190 .degree. C.).
The patent literature contains much information pertaining to the composition of the transfer print used for transferring the image onto a receiving substrate. Most of the literature pertaining to the substrate itself, on the other hand, does not deal with mugs or similar hard-surface articles; rather it understandably describes textiles such as cotton, polyester and wool because the popularity of imprinting these materials supersedes that of articles like coffee mugs.
U.S. Pat. Nos. 4,701,502, 4,824,925 and No. 4,923,934 disclose various coating compositions of general application. U.S. Pat. No. 4,943,684 describes a process for imprinting cylindrical ceramic and glass articles, specifically coffee mugs, via sublimation or heat (thermal) transfer printing. The patent also discloses epoxy and polyester polymers as the suitable coatings for the invention.
Commercially available mugs for personalized imprinting via sublimation or thermal transfer printing have a polymeric coating applied to the outer surface. The coating is intended to provide a stable layer for fixing the image to the surface of the mug and protecting it from erosion and abrasion during use. All of these coatings belong to the generic classes of polymers comprising epoxies, acrylics, polyesters and polyurethane. Polyurethanes provide the best image quality, but suffer in adhesion and are easily removed by repeated dishwasher cycles. The only types of coating that survive repeated washings in a mechanical dishwasher are based on epoxy material, which has a high glass transition temperature and therefore is not suitable for optimum image transfer. Thus, there is a need for a coating composition that combines the positive attributes of both types of polymers.
In my effort to combine the image-transfer quality of polyurethanes with the strong adhesion properties of epoxies, I discovered that the ideal polymer coating should exhibit two distinct glass transition temperatures before and after the image transfer process. The glass transition temperature, also called the softening temperature of a polymer, refers to the temperature at which the polymer changes from a glassy or brittle condition to a liquid or rubbery one. For an amorphous polymer, this is a narrow temperature region in which the polymer changes from a viscous or rubbery condition at temperatures above this region to a hard and relatively brittle one below it. This transformation is equivalent to the solidification of a liquid to a glass; it is not a phase transition. Above the glass transition temperature, portions of the polymer chains, usually called segments, are comparatively unhampered by the interactions between neighboring chains. See Paul J. Flory, Principles of Polymer Chemistry, pages 57-57, Cornell University Press, 1953.
The glass transition temperature of the ideal polymer coating for image imprinting by heat transfer should be as low as possible before the transfer printing operation in order to allow the dyes to diffuse easily into the polymer matrix as they are given off by the transfer printing medium. After the image transfer, in order to preserve the quality of the transferred image during repeated dishwashing cycles, the coating on the receptor surface should have as high a glass transition temperature as possible to minimize dye diffusion in the coating during hot-water dishwashing that would cause image distortion and bleeding.
One approach is to have a coating that is cured in two steps. The initial cure is performed below 375.degree. F. (190.degree. C.) to provide a coating with a low glass transition temperature. The final cure takes place during the imprinting step where the mug is brought to 375.degree. F. or higher temperatures. This final heating raises the glass transition temperature of the coating, sealing the dyes in the polymer matrix and providing durability of the coating and colorfastness to the image during repeated dishwashing. This appears to be the approach adopted by some suppliers of coated mugs for sublimation-transfer imprinting. The glass transition temperature of the epoxy coating on the mugs before the imprinting step is about 200.degree. F. (93.degree. C.). After imprinting, the glass transition temperature is raised to about 255.degree. F. (124.degree. C.).
This particular approach has one drawback. Since the glass transition temperature increases with the degree of curing, which in turn is related to exposure to high temperatures, the initial curing must be limited to a narrow process window in order to keep the glass transition temperature low enough to achieve optimum image transfer during the sublimation process. Also, the rate of reaction and curing of the coating prior to the image transfer operation is only retarded, not completely inhibited. Therefore, during the summer months, when the southern part of the United States experiences outdoor temperatures of 110.degree.-120.degree. F. (43.degree.-49.degree. C.) and the temperature in unrefrigerated cargo compartments (such as used to transport nonperishable goods like coated mugs) approaches 160.degree. F. (71.degree. C.), the thermal reaction and curing of the polymer is accelerated during storage and transportation of coated articles, resulting in poor quality of the subsequent image transfer. Thus, these articles require refrigeration and special handling, which add considerable cost to the final product.
The object of the present invention is a novel approach based on providing a coating that possesses a low initial glass transition temperature for accepting an image by thermal transfer and that, after the image-transfer process, features a protective seal over the transferred dyes in the polymer matrix by forming a highly crosslinked and therefore impervious outer layer.